Вычислите значения выражений с обыкновенными дробями.

1. $\frac{1}{4} + \frac{1}{3} = \frac{3}{12} + \frac{4}{12} = \frac{7}{12}$ 2. $\frac{2}{5} + \frac{1}{2} = \frac{4}{10} + \frac{5}{10} = \frac{9}{10}$ 3. $\frac{1}{3} + \frac{1}{7} = \frac{7}{21} + \frac{3}{21} = \frac{10}{21}$ 4. $\frac{2}{3} + \frac{5}{9} = \frac{6}{9} + \frac{5}{9} = \frac{11}{9} = 1\frac{2}{9}$ 5. $\frac{3}{8} + \frac{1}{2} = \frac{3}{8} + \frac{4}{8} = \frac{7}{8}$ 6. $\frac{1}{14} + \frac{3}{7} = \frac{1}{14} + \frac{6}{14} = \frac{7}{14} = \frac{1}{2}$ 7. $\frac{5}{6} + \frac{2}{3} = \frac{5}{6} + \frac{4}{6} = \frac{9}{6} = 1\frac{3}{6} = 1\frac{1}{2}$ 8. $\frac{7}{15} - \frac{4}{15} = \frac{3}{15} = \frac{1}{5}$ 9. $\frac{17}{21} - \frac{5}{21} = \frac{12}{21} = \frac{4}{7}$ 10. $\frac{4}{5} - \frac{1}{3} = \frac{12}{15} - \frac{5}{15} = \frac{7}{15}$ 11. $\frac{3}{4} - \frac{4}{9} = \frac{27}{36} - \frac{16}{36} = \frac{11}{36}$ 12. $\frac{9}{14} - \frac{2}{7} = \frac{9}{14} - \frac{4}{14} = \frac{5}{14}$ 13. $\frac{5}{12} - \frac{1}{3} = \frac{5}{12} - \frac{4}{12} = \frac{1}{12}$